Tuning optical and thermoelectric properties of LaCoO₃: Partial substitution of La by isovalent Gd

In this study, we report the structural, hardness, optical, and high temperature thermoelectric properties of Gd-doped lanthanum cobaltite La_1−xGd_xCoO₃ (x = 0.0, 0.2, 0.4, 0.6). The samples have been prepared by solid-state reaction technique. The room temperature XRD studies revealed that the pristine sample (x = 0.0) possesses a distorted rhombohedral structure with an R-3c space group. The samples with x ≥ 0.2 show a structural phase transition and possess an orthorhombic structure with the Pnma space group. Structural and optical studies confirm the formation of the La_1-xGd_xCoO₃ compound. The electrical resistivity for all the studied compositions is found to be inversely proportional to temperature, which is a typical semiconductor behavior. However, the electrical resistivity was found to increase with increased Gd content in the material. The pure and Gd substituted samples show a crossover from negative to positive thermopower, thereby indicating the presence of both electrons and holes as charge carriers. The substitution of Gd has enhanced the thermopower at high temperatures. The highest thermopower of –345 μV/K was obtained at 310 K for the pristine sample. A significant drop in electrical resistivity (T > 400 K) has caused the power factor to rise, resulting in a higher power factor of 141 µW/mK² for the sample x = 0.4 at 755 K.